The National Airspace System (NAS) includes the airspace, navigation facilities and airports of the United States, along with their associated information, services, rules, regulations, policies, procedures, personnel and equipment. These NAS components may be shared among private, commercial, and military aviation. Manned aircraft and unmanned aircraft systems (UAS) (sometimes referred to as unmanned aerial systems (UAS), unmanned aerial vehicles (UAV), and remotely piloted aircraft (RPA)) may operate in the NAS under control of Federal Aviation Administration (FAA) regulations. For manned aircraft, the FAA may require pilots to monitor the surrounding airspace for possible intruding aircraft and act to avoid a collision (sometimes referred to as detect-and-avoid (DAA)). For many UAS to be permitted in the NAS, the FAA requires the UAS be capable of a level of safety (Equivalent Level of Safety (ELOS)) equivalent to the detect-and-avoid requirements for manned aircraft. In effect, the UAS is required to operate to the same safety standards as a manned aircraft on instrument flight rules (IFR). Hobbyist UAS may be exempt from ELOS requirements provided the hobbyist UAS weighs less than a specified amount, is flown in line-of-sight of the UAS operator, and is flown below a specified altitude.
A manned aircraft flight through the NAS typically begins and ends at an airport; the airport may be controlled (by a tower) or uncontrolled. On departure, the aircraft may operate in one of five of the six airspace classes (based in part on altitude) with different flight rules for each airspace class. For example, depending on the airspace class and flight conditions, communication between pilots and controllers may be required. While Operation of an aircraft is the responsibility of the pilot, air traffic controllers (ATC) may give instructions for sequencing and safety as needed. After a controlled flight becomes airborne, control passes from the tower ATC who authorized the takeoff to a Terminal Radar Approach Control (TRACON). Between sectors administered by TRACONs are 21 contiguous areas of the NAS above 18,000 feet (class A airspace). Each of the 21 areas is managed by an Air Route Traffic Control Center (ARTCC), and generally referred to as a “Center,” that provide control functions. The ARTCCs manage more than 690 ATC facilities with associated systems and equipment to provide radar and communication services to aircraft transiting the NAS. An aircraft is handed off from one Center to another until the aircraft descends near its destination, when control is transferred to the TRACON serving the destination, and ultimately to the tower ATC serving the destination airport. Some airports have no TRACON around them, and control goes directly to or from a Center. Some flights are low enough and short enough that control is kept within one or more TRACONs without ever being passed to Center.
The NAS is transitioning to a Next Generation Air Transportation System (“NextGen” system), a feature of which involves non-radar surveillance of aircraft that are equipped with GPS satellite-based navigation systems, and that continuously broadcast their location. Receivers integrated into the air traffic control system or installed aboard other aircraft may receive the broadcast signals to provide an accurate depiction of real-time aviation traffic, both in the air and on the ground. This feature, known as ADS-B (automatic dependent surveillance-broadcast) is intended to provide not only enhanced aircraft separation, but also to allow pilots to use more precise and efficient landing paths, saving time and fuel. The FAA has mandated partial implementation of ADS-B by 2020.
Thus, one benefit of ADS-B may be improved situational awareness: through its broadcast signals, ADS-B may enhance safety by making an aircraft “visible,” in real-time, to air traffic control and to other appropriately equipped ADS-B aircraft. However, ADS-B also provides traffic- and government-generated graphical weather information and other data through TIS-B and FIS-B. Traffic Information Services-Broadcast, (TIS-B), is a component of the ADS-B technology that provides free traffic reporting services to aircraft equipped with ADS-B receivers. TIS-B allows non-ADS-B transponder equipped aircraft that are tracked by radar to have their location and track information broadcast to ADS-B equipped aircraft. Flight Information Services-Broadcast (FIS-B), also is a component of ADS-B technology that provides free graphical National Weather Service products, temporary flight restrictions (TFRs), and special use airspace information enabling pilots to increase levels of safety in the cockpit and on the ground.
ADS-B consists of two different services, ADS-B Out and ADS-B In. ADS-B Out periodically broadcasts aircraft information, such as identification (e.g., through an aircraft call sign), current position, altitude, and velocity, for example. ADS-B In refers to the reception by aircraft ADS-B data including broadcasts from nearby aircraft as well as graphical weather data (from FIS-B and TIS-B). ADS-B Out relies on two avionics components—a high-integrity GPS navigation source and a datalink (ADS-B unit). There are several types of certified ADS-B data links, the most common of which operate at 1090 MHz, essentially a modified Mode S transponder, or at 978 MHz. (Mode S or mode “select,” is a way to interrogate a specific aircraft by using a distinct address, such as an aircraft address, to which only the specific aircraft will respond. In addition to an aircraft identification signal, the Mode S transponder may provide other useful flight information.) Thus, to achieve ADS-B Out capability at 1090 MHz, an aircraft need only have installed an appropriate transponder and a certified GPS position source.
Two aspects of ADS-B operations may be of concern to general, commercial, and military aviation entities; namely (1) a lack of anonymity and (2) a lack of encryption, which may compromise manned and unmanned aircraft security. One aspect of ADS-B is that its operation may remove anonymity for aircraft observing visual flight rule (VFR) aircraft operations. This is because the International Civil Aviation Organization (ICAO) specifically assigns a unique 24-bit transponder code to each aircraft to allow monitoring of that aircraft when within the service volumes of the Mode-S/ADS-B system. Thus, unlike Mode A/C transponders, there is no code “1200”/“7000” to provide casual anonymity (for example, for a VFR flight, 1200 is the standard transponder code used in the NAS when no other code has been assigned). Mode-S/ADS-B identifies the aircraft uniquely among all aircraft in the world, in a manner similar to that of a MAC number for an Ethernet card or the International Mobile Equipment Identity (IMEI) of a GSM phone. Another aspect of the ADS-B broadcast of aircraft data is that the broadcast occurs over unencrypted data links. This means that the content of ADS-B broadcasts can be read by anybody who has the ability to use relatively simple receiving equipment such as a software defined radio to access the ADS-B broadcast.